Benefits of a swap function?

c++ stl swap
Rob picture Rob · Jan 4, 2010 · Viewed 12.5k times · Source

Browsing through some C++ questions I have often seen comments that a STL-friendly class should implement a swap function (usually as a friend.) Can someone explain what benefits this brings, how the STL fits into this and why this function should be implemented as a friend?

Answer

Yacoby picture Yacoby · Jan 4, 2010

For most classes, the default swap is fine, however, the default swap is not optimal in all cases. The most common example of this would be a class using the Pointer to Implementation idiom. Where as with the default swap a large amount of memory would get copied, is you specialized swap, you could speed it up significantly by only swapping the pointers.

If possible, it shouldn't be a friend of the class, however it may need to access private data (for example, the raw pointers) which you class probably doesn't want to expose in the class API.

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